Some compositions of matter have the characteristic of absorbing invisible ultraviolet radiation and re-radiating the energy of the absorbed radiation in the visible portion of the electromagnetic spectrum. Absorption of invisible ultraviolet radiation and re-radiation of the energy as visible radiation is commonly called fluorescence. Illuminating paper currency of unknown origin with invisible ultraviolet radiation is a well known method of identifying counterfeit paper currency. Typically, counterfeit United States currency has a much greater fluorescence than genuine United States currency. This difference in the characteristic absorption of ultraviolet radiation has been useful as a means to detect counterfeit United States currency. Some countries have been known to mark a portion of their paper currency with a substance that is not ordinarily visible, but becomes visible when the material fluoresces in response to illumination by ultraviolet radiation. Accordingly, devices have been developed for commercial use which illuminate paper currency with ultraviolet radiation to enable a quick determination from the degree of fluorescence as to the genuineness of paper currency of unknown origin.
The ultraviolet illumination and fluorescence method of determining the genuineness of paper currency has become increasingly important with the development of improved methods of reproducing the engravings of paper currency. In the past counterfeit paper currency often would be of a lesser print quality than genuine paper currency. This evidence coupled with an unusually high fluorescence characteristic (in U.S. paper currency) was enough information for the user of the ultraviolet illumination method to make a reliable determination that a particular paper bill was counterfeit. Today unfortunately, because of the easy availability of improved printing techniques, the inferior printing quality of counterfeit paper currency sometimes cannot be detected by the nonexpert. Therefore, an accurate determination of the degree of fluorescence of paper currency has increased in importance.
In counterfeit United States paper currency there are varying degrees of fluorescence depending on the material used. As stated earlier, genuine U.S. paper currency has a relatively low level of fluorescence. In the past ultraviolet radiation devices have depended on the user's visual determination of the level of fluorescence to detect counterfeit currency. Such a method of detection is open to subjective influences on the user which reduce the accuracy and reliability of the ultraviolet radiation devices.